Singapore Journal of Scientific Research2010-006x2010-0051orgz10.3923/sjsres.2011.264.268El-GenidyMohammed3201113There are a lot of machines and devices need periodic examination and after the results of the examination is to exclude the non defective machines while the rest need to change one part or more of its damaged components or that close to expiration date, assume that the system has served carry out switching and replacement the damaged parts by new parts. In this research was to obtain the probability distribution of the number of defective machines in the system during the period of time t and which can calculate the average number of defective machines in the system.]]>Tadj, L. and G. Choudhury,2005Hur, S., J. Kim and C. Kang,2003N and T policy.]]>Kennedy, W.J., J.W. Patterson and L.D. Fredendall,2002Agnihothri, S.R. and R.S. Kenett,1995Buzacott, J.A. and J.G. Shanthikumar,1992Kella, O.,1989Ayanzadeh, R., E. Shahamatnia and S. Setayeshi,2009Kharat, M.U. and N.S. Chaudhari,2005Zhou, X., Y. Yang, H. Shan and Z. Wang,2010Wang, W., M. Tang, Y. Ren and J. Li,2010Wang, J., X. Sha, L. Sun and Z. Zhang,2009